1. Field of the Invention
The present invention relates to a programmable divider apparatus, and more particularly to a programmable divider apparatus that can selectively supply a feedback control.
2. Description of Prior Art
Reference is made to FIG. 1 which is a block diagram of a prior art frequency synthesizer. The frequency synthesizer 30 comprises a phase/frequency detector 300, a loop filter 302, a voltage-controlled oscillator 304, a frequency divider 306, and a programmable frequency divider 308.
The phase/frequency detector 300 compares a difference between a reference input frequency (Fref) and a feedback frequency outputted from the frequency divider 306, and outputs an output signal corresponding to phase difference and frequency difference for these two signals. The output signal is processed by the loop filter 302, and the processed output signal is coupled to the voltage-controlled oscillator 304. The voltage-controlled oscillator feedback frequency (Fvco) will be utilized to lock the reference input frequency (Fref) by frequency divider 306. The phase and frequency of the frequency synthesizer 30 are locked when the phase and frequency of the frequency divider output frequency (Fdiv) and the phase and frequency of the voltage-controlled oscillator feedback frequency (Fvco) are synchronous.
The frequency divider 306 can perform frequency division for the voltage-controlled oscillator 304 such that the reference input frequency (Fref) can be locked by the frequency synthesizer 30. Moreover, the output of the voltage-controlled oscillator 304 equals M multiplies Fref (M×Fref) when the divisor of the divider 306 equals M. Hence, the divisor M of the frequency divider 306 can be adjusted to produce an output frequency Fdiv based on the voltage-controlled oscillator feedback frequency (Fvco). If a divisor of the programmable frequency divider 308 equals P, an output frequency (Fout) of the frequency synthesizer 30 is repressed by:Fout=(M/P)×Fref
Hence, by setting parameters M and P for a frequency divider are designed to produce various frequency source according to a reference frequency Fref. A frequency synthesizer can function as a clock generator or frequency source and is developed for application elements in the clock circuit. FIG. 2 is a block diagram of an application for a frequency synthesizer 30, which is applied to a reception circuit for a mobile phone. In conclusion, the adjustable divisor of the programmable frequency divider is an important component in a phase-lock-loop circuit. Hence, the phase-lock-loop circuit has a more versatile function if a programmable multi-divisor frequency divider can be implemented in a simplified way.